I have come across this situation myself quite a few times; Have you?
You go to a new site & the on-site admins are having difficulty in maintaining a precise inventory of their VM's, lets be honest... Maintaining a spreadsheets can be a chore. One answer would be to use V-Scout! the folks over at Embotics make V-Scout available for free and what great little tool it is.
If you are unfamiliar with it's certainly worth checking out, it could save you a massive amount of time over maintaining the dreaded spreadsheet.
http://www.embotics.com/products/v-scout
What do you use?